Examinations are carried out with the aid of computed tomography scanners for many problems in the field of medicine. In this case, a CT scanner includes an X-ray source as well as an X-ray detector which is located opposite the X-ray source and includes a number of detector modules which are formed by individual detector elements. The detector elements are generally arranged in one or more rows alongside one another in order to achieve position-resolved detection of the X-ray radiation. Knowledge about the distribution of the materials within the object being examined can be obtained from the position-resolved measurement of the attenuation of the X-ray radiation caused by the object being examined which is located between the X-ray source and the detector.
In order to detect X-ray radiation, detectors with indirect transducers are known on the one hand, which are composed of a scintillator material, followed by a photodetector. The scintillator converts the incident X-ray radiation to visible radiation, which is then detected by the photodetector. The number of photons produced per X-ray quantum is in this case in general approximately proportional to their quantum energy. As a rule, the electrical signal obtained from the photodetector is integrated over a predetermined time period when using this technique.
Furthermore, specific semiconductor materials are also known for detection of X-ray radiation, in which the incident X-ray radiation produces charge carriers directly. The number of charge carriers produced in these direct transducers per X-ray quantum is in this case in general approximately proportional to their quantum energy.
A counting method is also known for CT scanners, instead of integrating the electrical signal obtained from the transducer. By way of example, DE 102 12 638 A1 discloses a detector module for a CT scanner, which has a number of detector elements which detect the X-ray radiation on the basis of direct transducers. Each detector is connected to a pulse generator in order to produce counting pulses as a function of the received electrical signals. The pulse generator passes the pulses to a counting device, which counts the counting pulses over a time period which can be predetermined, and outputs the result.
When using counting radiation detectors in areas in which the number of radiation quanta to be detected per unit time is very high, the measured counting rate may be corrupted owing to the dead time of the counting detector.
In this context, the expression dead time refers to the time period which the detector requires for processing an event after it occurs. All the other events which arrive during this time period are lost. Particularly at the high rates at which X-ray ray quanta arrive at the detector elements during X-ray CT scanning, the dead time of the detector elements leads to the linearity condition for CT scanning not being satisfied, and thus to the image quality not satisfying the requirements.